It’s hard to picture how the Earth’s climate will develop over the next few decades. We know that global warming is going up nonlinearly and that every 0.1°C of warming will cause more damage and suffering than the previous one. Still, it’s hard to imagine how much the human cost will be for different levels of global warming.

A recent study managed to put a finger on the human cost of global warming and mapped out where climate change is happening and who is most affected. And by doing so, the authors make a compelling case for immediate and aggressive policies to prevent warming above 1.5°C.

Current policies and commitments put us on track for ~2.7°C of warming at the end of this century, which would have catastrophic consequences for people and ecosystems. In a worst-case scenario, where we fail to meet our targets or where we cross tipping points and activate feedback loops that result in a permanent shift away from the current climate, we could even put the lives and livelihoods of billions of people at risk.

In this study – and the two before it – the authors looked at the climate conditions in which human societies have thrived, and they found that most people live in areas with a Mean Annual Temperature (MAT) spread across 13°C or 25°C. Conditions outside those are too cold, dry, or hot and are associated with higher death rates and lower food production. But now that global temperatures are increasing, people are pushed outside this ‘human climate niche’.

As our planet heats up, an increasing number of people will be experiencing what scientists call ‘unprecedented heat’ (MAT≥29°C). Unprecedented heat is linked to decreased labour productivity, cognitive performance and learning, adverse pregnancy outcomes, reduced crop yield, increased mortality, conflict, and spread of infectious diseases.

Today, ~30 million people are exposed to unprecedented heat, but that number is quickly growing. Affected areas now only cover 0.8% of the global land surface (black) but are projected to cover 19% in 2070 in a worst-case scenario (dark green). Our current trajectory is projected to push 2 billion people outside the climate niche and force a billion or more to migrate to cooler places.

Even if people migrate to cooler areas, they will still experience more frequent heatwaves, droughts and other extreme weather events. A neighbouring region or country might have a MAT that’s within the human climate niche, but whether it’s 27°C or 28°C, it’s still incredibly hot.

People are most comfortable with dry air temperatures between 22–26°C in the shadow. Above 28°C, well-being quickly declines. Outside the human comfort zone, we must adapt by altering clothing, changing our environment, and altering work patterns. However, there is a limit to adaptation; at a certain point, sweating doesn’t cool us down anymore, and temperatures above 40°C can even be fatal – the risk increases with humidity. Other mammals have similar physiological limits to humans.

Climate change also affects us indirectly through the resources that sustain us. Higher temperatures favour vectors of human disease, increase the spread of key crop pests and pathogens, decrease crop yield potential, and limit access to (irrigation) water, which are, in turn, linked to famine, economic inequality, migration, governance failures, and conflict.

When the discussed extreme heat projections are overlapped with the 2021 Fragile States Index or with population density (check the resources for both), it’s evident that this is a future that we must avoid at all costs. This is more than a climate crisis; this is a polycrisis.

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